What if cameras were rated by true ISO and lenses by t-stop?

http://www.dpreview.com/news/2012/04/16/carl-zeiss-launches-70-200-compact-tele-cine-lens

 

The t-stop of almost all SLR lenses is not worst than the geometric
aperture - at least not enough to matter. A very few SLR lenses have a
t-stop that is worse, and in the case of apodization filtered lenses,
variable t-stops.

It would be nice if the "natural" (unity gain) ISO of the camera were
declared. eg: on my camera which begins at ISO 100, the unity gain ISO
appears to be somewhere around 150 (looking at noise graphs; various
sources).

 

It matters enough for video production - particularly when different
lenses will be used in multi-camera scenes. If there's a visible
exposure difference between cameras/lenses, an extra layer of complexity
is added to the editing process. For still photos, it would make no
difference to the intended output, or is trivial to adjust in PP.

 

I've seen incidents where claimed apertures between two lenses produced
different results. but mostly with older versus newer lenses.

 

That would be a good place to set it. But like audio equipment in the
1970's the camera mfgs will have to be "encouraged" to adhere to a
standard.

 

The amount is small is SLR lenses (say 1/2 stop). In cine lenses, the
difference between aperture and t-stop can be several stops. What
you're describing and the differences if SLR claimed aperture, actual,
light loss, and variance are different issues and much smaller.